arctic plants and animals, and these would be nearly the same with
those of Europe; for the present circumpolar inhabitants, which we
suppose to have everywhere travelled southward, are remarkably uniform
round the world. We may suppose that the Glacial period came on a
little earlier or later in North America than in Europe, so will the
southern migration there have been a little earlier or later; but this
will make no difference in the final result.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending higher and higher, as the warmth increased,
whilst their brethren were pursuing their northern journey. Hence, when
the warmth had fully returned, the same arctic species, which had
lately lived in a body together on the lowlands of the Old and New
Worlds, would be left isolated on distant mountain-summits (having been
exterminated on all lesser heights) and in the arctic regions of both
hemispheres.
Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the migration as the
cold came on, and the re-migration on the returning warmth, will
generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Ramond, are more especially
allied to the plants of northern Scandinavia; those of the United
States to Labrador; those of the mountains of Siberia to the arctic
regions of that country. These views, grounded as they are on the
perfectly well-ascertained occurrence of a former Glacial period, seem
to me to explain in so satisfactory a manner the present distribution
of the Alpine and Arctic productions of Europe and America, that when
in other regions we find the same species on distant mountain-summits,
we may almost conclude without other evidence, that a colder climate
permitted their former migration across the low intervening tracts,
since become too warm for their existence.
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